mastering is usually done by people with equipment we can't afford, in rooms we can't afford, with experience with said equipment and rooms that we don't have.
you mix to the extent of the the ability and limits of your ear and equipment, then someone who has the training to optimize the levels (compression, limiting) and frequencies (eq) for the greatest common denominator of listener and listening device "masters" it. so then it not only sounds good in your studio, but also on the radio, in the the car, in the ipod, the boombox, the desktop etc...